The correct answer is B. 25 cm.
The distance of distinct vision (DDV) is the distance at which an object can be seen clearly without accommodation. For a normal human eye, the DDV is 25 cm. This means that if an object is placed at a distance of 25 cm from the eye, it will be seen clearly. If the object is placed closer or further than 25 cm, it will be seen out of focus.
The DDV is determined by the length of the eyeball and the flexibility of the lens. The length of the eyeball is fixed, but the lens can be adjusted to focus on objects at different distances. When an object is placed at a distance of 25 cm, the lens is in its relaxed state and the image is focused on the retina. If the object is placed closer than 25 cm, the lens has to be contracted to focus the image on the retina. If the object is placed further than 25 cm, the lens has to be relaxed to focus the image on the retina.
The DDV can be affected by a number of factors, including age, eye diseases, and medications. With age, the lens loses its flexibility and the DDV decreases. This is why people who are over 40 often need reading glasses. Eye diseases, such as nearsightedness and farsightedness, can also affect the DDV. Nearsighted people can see objects that are close to them clearly, but objects that are far away are out of focus. Farsighted people can see objects that are far away clearly, but objects that are close to them are out of focus. Medications, such as those used to treat glaucoma, can also affect the DDV.
If you are experiencing any problems with your vision, it is important to see an eye doctor. The doctor can determine if you have any eye problems and recommend treatment.